1. Field of the Invention
The present invention relates to a surface acoustic wave device having an interdigital transducer (IDT) electrode and a SiO2 film for improving temperature characteristics on a piezoelectric substrate and to a method for manufacturing the same. More specifically, the present invention relates to a method for manufacturing a surface acoustic wave device, which includes a step of forming a wiring pattern on a piezoelectric substrate, and also relates to a surface acoustic wave device having an improved wiring pattern structure on the piezoelectric substrate.
2. Description of the Related Art
A transmitting frequency band of a portable telephone and a receiving frequency band thereof are closely allocated. A surface acoustic wave filter is widely used as a band-pass filter of an RF stage in a portable telephone. Since the transmitting frequency band and the receiving frequency band are closely allocated, the surface acoustic wave filter used for the band-pass filter of the RF stage in a portable telephone is required to have a good frequency temperature characteristic, in particular, to have a small change in frequency with temperature.
According to such a requirement, for example, Japanese Unexamined Patent Application Publication No. 2004-254291 discloses a structure in which a film for improving frequency temperature characteristics made of SiO2 is formed on a piezoelectric substrate. As shown in a plan view of FIG. 7A and a cross-sectional view of FIG. 7B that is taken along line A-A of FIG. 7A, a surface acoustic wave filter 101 includes an IDT electrode 103 and reflectors 104 and 105 provided on a piezoelectric substrate 102. A SiO2 film 106 covers the IDT electrode 103 and the reflectors 104 and 105.
In order to electrically connect the IDT electrode 103 to the outside, a hole must be provided in the SiO2 film and an exposed portion of the electrode in the hole is electrically connected to the outside using a bonding wire or other suitable connection structure.
Recently, the circuit structure of surface acoustic wave filter devices has become very complicated. That is, in order to magnify an attenuation function or achieve a balance-unbalance conversion function, the surface acoustic wave devices include structures in which a plurality of surface acoustic wave filters are connected or a surface acoustic wave filter is connected to a surface acoustic wave resonator.
For example, Japanese Unexamined Patent Application Publication No. 2004-282707 discloses a surface acoustic wave filter device having a circuit structure shown in FIG. 8. As shown in FIG. 8, in a surface acoustic wave filter device 111, a first and a second longitudinally coupled resonator surface acoustic wave filters 113 and 114 are connected to an unbalanced terminal 112 in parallel. The longitudinally coupled resonator surface acoustic wave filter 113 is connected to a balanced terminal 117 through a one port surface acoustic wave resonator 115. The second longitudinally coupled resonator surface acoustic wave filter 114 is connected to a second balanced terminal 118 through a one port surface acoustic wave resonator 116.
Japanese Unexamined Patent Application Publication No. 2004-282707 discloses a wiring pattern for achieving the above-mentioned circuit structure, which is shown in FIG. 9 as a schematic plan view. According to the schematic plan view, a piezoelectric substrate 121 includes the unbalanced terminal 112, the first and the second balanced terminals 117 and 118, the first and the second longitudinally coupled resonator surface acoustic wave filters 113 and 114, the one port surface acoustic wave resonators 115 and 116 provided thereon and these components are connected with each other using wiring patterns.
If the wiring patterns are arranged to provide the connection, the wiring patterns which are connected to different potentials may cross each other. For example, in FIG. 9, a wiring pattern 122 electrically connecting the first longitudinally coupled resonator surface acoustic wave filter 113 to the one port surface acoustic wave resonator 115 is crossed with a wiring pattern 123 connecting the central IDT of the longitudinally coupled resonator surface acoustic wave filter 113 to the ground potential. In this case, in order to prevent a short circuit between the wiring patterns 122 and 123, an insulating layer 124 made of a photosensitive resin is provided therebetween.
Similarly, a wiring pattern 125 connecting the second longitudinally coupled resonator surface acoustic wave filter 114 to the one port surface acoustic wave resonator 116 and the wiring pattern 126 connecting the central IDT of the second longitudinally coupled resonator surface acoustic wave filter to a ground potential are three-dimensionally crossed with each other with an insulating layer 127 made of a photosensitive resin provided therebetween.
FIG. 10 is a cross-sectional view of the three-dimensional crossing portion taken along line B-B in FIG. 9.
As clearly shown in FIG. 9, the surface acoustic wave filter device 111 has, as mentioned above, the wiring patterns 123 and 126 connected to the ground potential, the wiring patterns 122 and 125 electrically connecting the surface acoustic wave filter to the surface acoustic wave resonator, and the wiring patterns connecting the unbalanced terminal and the first and the second balanced terminals to other electrode portions. These wiring patterns provided on the piezoelectric substrate 121 make the structure thereof complicated. Therefore, the above-mentioned three-dimensional crossing portions are formed at several positions.
As mentioned above, the surface acoustic wave filter described in Japanese Unexamined Patent Application Publication No. 2004-254291 includes the SiO2 film 106 covering the IDT electrode on the piezoelectric substrate in order to improve the frequency temperature characteristic.
On the contrary, the surface acoustic wave filter device 111 described in Japanese Unexamined Patent Application Publication No. 2004-282707 includes the insulating layers 124 and 127 made of photosensitive resin, which are formed by photolithography. These insulating layers are formed to prevent short circuiting between the wiring patterns at the crossing portions of the wiring patterns on the piezoelectric substrate, in particular, at the crossing portions where the wiring patterns connected to different potentials cross each other.
Therefore, in the surface acoustic wave filter device used in a RF stage of a portable telephone, a SiO2 film is preferably provided to improve a frequency temperature characteristic, and if the wiring patterns become complicated, an insulating layer for preventing short circuiting between wiring patterns should be formed thereon in addition to the SiO2 film by patterning photosensitive resin layer. This complicates the manufacturing process of surface acoustic wave devices and the cost thereof is significantly increased.
Furthermore, when the photosensitive resin is applied and then patterned, the photosensitive resin must be heated to a high temperature in order to be cured. This also leads to deterioration of the surface of the piezoelectric substrate caused by the high temperature, and the desired characteristics are rarely obtained.
Furthermore, if the IDT electrodes are made of corrodable metals such as Cu, the IDT electrodes are easily corroded by a developer used for patterning of the photosensitive resin.